This essay identifies and reviews the major and easily recognizable characteristics of a library discourse and proposes models describing the actual, possible, and necessary modes of library communication. The transmittal of orderly thoughts or information in a library situation differs from a conventional communication pattern. Three unique phases can be distinguished in the library's communication, roughly corresponding to the traditional classification of logical propositions and their basic assertions: (1) the procedural or actual; (2) the contextual or possible; and (3) the conceptual or necessary conditions of any library discourse. The resulting three-fold modality of a library discourse interrelates the physical and objective reality of the message communicated through its carrier, with the conceptual and subjective reality as it is perceived by the recipient of that message. (Author/CWM)